Reducing Inequalities in Vaccine uptake in the European Region – Engaging Underserved communities (RIVER-EU) is a 5-year (2021-2026) project funded by the Horizon 2020 Research Programme whose main goal is to collect evidence on health system determinants of high and low vaccine uptake in eight specific contexts, which will be used to identify and/or develop interventions to reduce barriers to vaccine uptake in underserved communities and increase trust in the health system.